Randall M. Stoltenberg

Evaluation of Solution-Processed Reduced Graphene Oxide Films as Transparent Conductors

Processable, single-layered graphene oxide (GO) is an intriguing nanomaterial with tremendous potential for electronic applications. We spin-coated GO thin-films on quartz and characterized their sheet resistance and optical transparency using different reduction treatments. A thermal graphitization procedure was most effective, producing films with sheet resistances as low as 10(2) -10(3) Omega/square with 80% transmittance for 550 nm light. Our experiments demonstrate solution-processed GO films have potential as transparent electrodes.

Highly sensitive flexible pressure sensors with microstructured rubber dielectric layers

The development of an electronic skin is critical to the realization of artificial intelligence that comes into direct contact with humans, and to biomedical applications such as prosthetic skin. To mimic the tactile sensing properties of natural skin, large arrays of pixel pressure sensors on a flexible and stretchable substrate are required. We demonstrate flexible, capacitive pressure sensors with unprecedented sensitivity and very short response times that can be inexpensively fabricated over large areas by microstructuring of thin films of the biocompatible elastomer polydimethylsiloxane. The pressure sensitivity of the microstructured films far surpassed that exhibited by unstructured elastomeric films of similar thickness, and is tunable by using different microstructures. The microstructured films were integrated into organic field-effect transistors as the dielectric layer, forming a new type of active sensor device with similarly excellent sensitivity and response times.

Solution coating of large-area organic semiconductor thin films with aligned single-crystalline domains

Solution coating of organic semiconductors offers great potential for achieving low-cost manufacturing of large-area and flexible electronics. However, the rapid coating speed needed for industrial-scale production poses challenges to the control of thin-film morphology. Here, we report an approach--termed fluid-enhanced crystal engineering (FLUENCE)--that allows for a high degree of morphological control of solution-printed thin films. We designed a micropillar-patterned printing blade to induce recirculation in the ink for enhancing crystal growth, and engineered the curvature of the ink meniscus to control crystal nucleation. Using FLUENCE, we demonstrate the fast coating and patterning of millimetre-wide, centimetre-long, highly aligned single-crystalline organic semiconductor thin films. In particular, we fabricated thin films of 6,13-bis(triisopropylsilylethynyl) pentacene having non-equilibrium single-crystalline domains and an unprecedented average and maximum mobilities of 8.1±1.2 cm(2) V(-1) s(-1) and 11 cm(2) V(-1) s(-1). FLUENCE of organic semiconductors with non-equilibrium single-crystalline domains may find use in the fabrication of high-performance, large-area printed electronics.

Ambipolar, high performance, acene-based organic thin film transistors.

We present a high performance, ambipolar organic field-effect transistor composed of a single material. Ambipolar molecules are rare, and they can enable low-power complementary-like circuits. This low band gap, asymmetric linear acene contains electron-withdrawing fluorine atoms, which lower the molecular orbital energies, allowing the injection of electrons. While hole and electron mobilities of up to 0.071 and 0.37 cm2/V.s, respectively, are reported on devices measured in nitrogen, hole mobilities of up to 0.12 cm2/V.s were found in ambient, with electron transport quenched. These devices were fabricated on octadecyltrimethoxysilane-treated surfaces at a substrate temperature of 60 degrees C.

Solution-processed, high-performance n-channel organic microwire transistors

The development of solution-processable, high-performance n-channel organic semiconductors is crucial to realizing low-cost, all-organic complementary circuits. Single-crystalline organic semiconductor nano/microwires (NWs/MWs) have great potential as active materials in solution-formed high-performance transistors. However, the technology to integrate these elements into functional networks with controlled alignment and density lags far behind their inorganic counterparts. Here, we report a solution-processing approach to achieve high-performance air-stable n-channel organic transistors (the field-effect mobility (μ) up to 0.24 cm2/Vs for MW networks) comprising high mobility, solution-synthesized single-crystalline organic semiconducting MWs (μ as high as 1.4 cm2/Vs for individual MWs) and a filtration-and-transfer (FAT) alignment method. The FAT method enables facile control over both alignment and density of MWs. Our approach presents a route toward solution-processed, high-performance organic transistors and could be used for directed assembly of various functional organic and inorganic NWs/MWs.

Strong and Stable Doping of Carbon Nanotubes and Graphene by MoOx for Transparent Electrodes

MoO(x) has been used for organic semiconductor doping, but it had been considered an inefficient and/or unstable dopant. We report that MoO(x) can strongly and stably dope carbon nanotubes and graphene. Thermally annealed MoO(x)-CNT composites can form durable thin film electrodes with sheet resistances of 100 Ω/sq at 85% transmittance plain and 85 Ω/sq at 83% transmittance with a PEDOT:PSS adlayer. Sheet resistances change less than 10% over 20 days in ambient and less than 2% with overnight heating to 300 °C in air. The MoO(x) can be easily deposited either by thermal evaporation or from solution-based precursors. Excellent stability coupled with high conductivity makes MoO(x)-CNT composites extremely attractive candidates for practical transparent electrodes.

Direct Patterning of Gold Nanoparticles Using Dip-Pen Nanolithography

Various methods for the patterned assembly of metal nanoparticles have been developed in order to harness their unique electrical and optical properties for device applications. This paper discusses a method for direct writing of Au nanoparticles at nanoscale resolution using dip-pen nanolithography. First, a procedure was developed for increasing the loading of Au nanoparticles onto AFM tips to prolong patterning life. AFM tips were subsequently imaged by scanning electron microscopy to determine ink coverage and to gain insight into the deposition process. Next, surface interactions, relative humidity, and writing speed were controlled to determine an optimal range of conditions for deposition. Various ink-substrate combinations were studied to elucidate the dependence of deposition on interactions between Au nanoparticles and the substrate surface; inks consisted of positively and negatively charged particles, and substrates were SiO(2) surfaces modified as hydrophilic or hydrophobic and interacted electrostatically or covalently with Au nanoparticles. Results indicate that a highly hydrophilic surface is required for Au nanoparticle deposition, unless covalent binding can occur between the Au and substrate surface. The optimal range of relative humidity for patterning was found to be 40-60%, and Au nanoparticle deposition was not sensitive to writing speeds ranging from 0.01 to 2 microm/s.

Fabrication and evaluation of solution-processed reduced graphene oxide electrodes for p- and n-channel bottom-contact organic thin-film transistors.

Reduced graphene oxide (RGO) is an electrically conductive carbon-based nanomaterial that has recently attracted attention as a potential electrode for organic electronics. Here we evaluate several solution-based methods for fabricating RGO bottom-contact (BC) electrodes for organic thin-film transistors (OTFTs), demonstrate functional p- and n-channel devices with such electrodes, and compare their electrical performance with analogous devices containing gold electrodes. We show that the morphology of organic semiconductor films deposited on RGO electrodes is similar to that observed in the channel region of the devices and that devices fabricated with RGO electrodes have lower contact resistances compared to those fabricated with gold contacts. Although the conductivity of RGO is poor compared to that of gold, RGO is still an enticing electrode material for organic electronic devices possibly owing to the retention of desirable morphological features, lower contact resistance, lower cost, and solution processability.

Organic Transistors with Ordered Nanoparticle Arrays as a Tailorable Platform for Selective, In Situ Detection

The use of organic transistors as sensing platforms provides a number of distinct advantages over conventional detection technologies, including their tunability, portability, and ability to directly transduce binding events without tedious and expensive labeling procedures. However, detection efforts using organic transistors lack a general method to uniquely specify and detect a target of interest. While highly sensitive liquid- and vapor-phase sensors have been previously reported, detection has been restricted either to the serendipitous interaction of the analyte molecules with the organic semiconductor or to the covalent functionalization of the semiconductor with receptor groups to enhance specificity. However, the former technique cannot be regularly relied upon for tailorable sensing while the latter may result in unpredictable decreases in electronic performance. Thus, a method to provide modular receptor sites on the surface of an organic transistor without damaging the device will significantly advance the field, especially regarding biological species detection. In this work, we utilized a block copolymer to template ordered, large-area arrays of gold nanoparticles, with sub-100 nm center-to-center spacing onto the surface of an organic transistor. This highly modular platform is designed for orthogonal modification with a number of available chemical and biological functional groups by taking advantage of the well-studied gold-thiol linkage. Herein, we demonstrate the functionalization of gold nanoparticles with a mercury-binding oligonucleotide sequence. Finally, we demonstrate the highly selective and robust detection of mercury(II) using this platform in an underwater environment.

Driving High-Performance n- and p-type Organic Transistors with Carbon Nanotube/Conjugated Polymer Composite Electrodes Patterned Directly from Solution

We report patterned deposition of carbon nanotube/conjugated polymer composites from solution with high nanotube densities and excellent feature resolution. Such composites are suited for use as electrodes in high-performance transistors of pentacene and C(60), with bottom-contact mobilities of > 0.5 and > 1 cm(2) V(−1) s(−1), respectively. This represents a clear step towards development of inexpensive, high-performance all-organic circuits.